Hunting the Wild Nutrinos

Hunting the Wild Nutrinos

For a project with such grand hopes, the technology it uses is fairly simple. The detectors are commercial photomultiplier tubes packed inside thick glass globes. They are like “flashlights in reverse,” explains Francis Halzen, an astrophysicist at the University of Wisconsin-Madison and one of the leaders of the project. The tubes can detect the faintest traces of light given off by neutrino collisions, amplify them more than 100 million times, and transmit the signals back to the surface as electrical pulses through coaxial cables for analysis.

To drill the holes for the strings of detectors, scientists use a technique borrowed from glaciologists called hot-water drilling. The rig uses a device similar to a massive shower head that gushes scalding water to melt the ice. As the ice melts, the water is recirculated, heated, and fed back down the hole. Since the drill is steered by gravity, its fall is straight, deviating less than 1 meter over a kilometer. Because of the ice’s natural insulating properties, the hole doesn’t refreeze for about four days, giving the crews enough time to deploy the strings of detectors.

The detectors are already hard at work. They record some 25 events per second, but nearly all are too weak to be of interest to Amanda scientists. The researchers are looking for the signature of neutrinos that have traveled all the way through the earth to arrive at detectors from below, figuring that the earth will filter out the radiation from less energetic sources and leave only the signature of high-energy neutrinos. In fact, Halzen says the detectors have picked up such readings, which the collaborators believe are traces of the high-energy neutrinos the project is being built to detect.

To measure actual energy levels more precisely, Halzen makes use of the very air bubbles in the ice that originally seemed to pose a problem. It turns out that the bubbles gather and scatter light in a predictable fashion. By measuring how far and in what direction the light emitted by excited muons scatters throughout the detector array, scientists can calculate the energy levels of incoming neutrinos.

In his public talks, Halzen likes to point out how astronomy has succeeded in exploring radiation arriving from space at wavelengths spanning 18 orders of magnitude, from languid radio waves to intense gamma rays. Neutrinos-which are emitted by radiation with wavelengths up to eight orders of magnitude smaller than gamma rays-are in essence a messenger that astronomers hope to use to describe that radiation. It’s hard to know what energy sources astronomers would find at that level, but, Halzen says, “it’s inconceivable that there’s nothing out there.”

Ironically, one fear of the Amanda scientists is that their findings might put them too far ahead of the rest of the astrophysics community. As Halzen says, if no other instrument is able to replicate Amanda’s observations, “I’m afraid one day we’ll see all these wonderful things and no one will believe us.”

Scientists familiar with the project, however, are expecting a better fate. “Our experience in astronomy in the last 50 years is that when a new technique for observing the universe is applied, nature always turns out to have surprises in store for us,” says John Bahcall, a physicist who studies neutrinos at the Institute for Advanced Study in Princeton, N.J. “What we observe,” he says, “is not what we expect to observe.”